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I'm an undergrad interested in astophysics and this question struck my mind:

As the CNO cycle requires carbon to start, and carbon is produced by thermonuclear fusion in stars, is it fair to think that the hotter stars in the first generation didn't have a CNO cycle?
Or should I instead think that the first generation of stars had an initial metalicity and therefore the CNO cycle was possible?

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First generation stars didn't do the CNO cycle initally.

It is estimated, that after the Big Bang three quarters of the matter is hydogren, one quarter helium, and trace amounts of heavier elements. This is then also the composition of (as of now unobserved) first generation stars, having zero metallicity. Even though those stars probably were very massive, they could not run the CNO cycle initially.

Luckily, the CNO cycle is not the only nuclear fusion process in stars. In the proton-proton chain reaction, hydrogen is fused into helium. The star is in equilibrium until the hydrogen in the core is "used up". Then it may collapse and heat until the triple-alpha process begins to produce carbon. From there on there are more and more processes producing many different elements. Due to convection and shell-burning, the CNO cycle might have been running in those first generation stars at later stages.

Interestingly, the most massive of the first generation (population III) stars may have ended up as black holes, gobbling up all of the produced metals (elements heavier than helium). Many others may have exploded in pair-instability supernovae ejecting all material into interstellar space, leaving no compact remnant behind.

A Heger and S. E. Woosley: The Nucleosynthesis Signature of Population III

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  • $\begingroup$ the proton proton chain is mainly present on starts that are not so massive , so those massive stars from the first generation probably collapsed rather quickly by not being able to run the cno chain right? (I know you said initially but I'm thinking probably the most massive didn't live long enough to start ) $\endgroup$
    – Gerardo Suarez
    Commented Mar 30, 2018 at 18:37
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    $\begingroup$ @GerardoSuarez Not right. It just means their cores would need to be a little bit hotter to get the same fusion rate from the pp chain. $\endgroup$
    – ProfRob
    Commented Mar 30, 2018 at 23:00
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    $\begingroup$ The statement "the pp chain is mainly present in stars that are not so massive" is valid in our present day star population only, where every star has some metallicity. First generation stars may not have been running the CNO cycle predominantly, but that doesn't necessarily mean they collapsed quickly, because other processes may have dominated after the pp chain. $\endgroup$
    – Hannes
    Commented Mar 31, 2018 at 11:07
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    $\begingroup$ This thesis is a recent description and modeling of very low metallicity (pop 3), high mass stars. It's over 150 pages long, and I haven't been able to parse a significant fraction of it yet. From what I have gathered so far, it seems the idea that the triple alpha process leads to the CNO cycle occurring (on some non-trivial level) is supported. There's another paper from much longer ago that suggests the much more limited CNO reactions may have impacted pop 3 supernovas. $\endgroup$
    – zibadawa timmy
    Commented Mar 31, 2018 at 16:44
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I guess by what you mean "first generation" is the Pop III (aka zero-metallicity) stars.

These stars composed of H and He only (might have very small and negligible about of heavier elements) since they formed at the very early universe. Their evolution starts with fusion of H to get more He, and burning He to get more heavier elements. This is similar to what happens in the Sun. Therefore, carbon can be produced, and CNO cycle can occur at some point of the life time.

This is also supported by the idea the idea that Pop III might be very massive and maintain its mass without significant loss by the wind mechanism (i.e., mass loss rate is higher with metallicity due to line emission of heavy elements). It has been shown by simulations that these massive stars can end their lives by thermonuclear explosion (i.e., similar to SNe Type Ia), instead of core-collapse one.

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